Wavelength division multiplexing (WDM) is a valuable technique for increasing the information carrying capacity of optical transmissions for voice communications as well as high density transmission of data. In essence, WDM involves modulating light beams of multiple discrete wavelengths with information to be transmitted, combining or multiplexing the beams into a single polychromatic light beam, and transmitting the polychromatic beam to a receiving location by means, for example, of optical fibers or waveguides. At the receiving location, the beam is demultiplexed or separated back into its component discrete wavelength beams, each of which may then be demodulated to extract the information carried by the beam. Thus, many channels of information can be transmitted simultaneously thereby multiplying the information carrying capacity of the transmission.
Wavelength division optical transmission requires an optical multiplexer for combining individual optical signals into a multiplexed signal and an optical demultiplexer for separating the multiplexed signal back into its discrete wavelength components. A variety of optical multiplexers and demultiplexers have been developed for this purpose, many of which are for use in the telecommunications industry. Some of these devices make use of optical gratings because such gratings inherently diffract and/or reflect light beams of different wavelengths at different angles. For example, U.S. Pat. No. 6,011,884 of Dueck et al. discloses an optical wavelength division multiplexer that integrates an axial gradient refractive index element with a diffraction grating. Enhanced efficiency multiplexing of discrete wavelength optical beams into a single polychromatic beam for transmission is asserted. U.S. Pat. No. 4,923,271 of Henry et al. discloses an optical multiplexer/demultiplexer having a plurality of focusing Bragg reflectors, each including a plurality of confocal elliptical grating lines. U.S. Pat. No. 5,818,986 of Asawa et al. discloses an optical wavelength demultiplexer incorporating angular back reflection from a series of Bragg gratings in the optical signal path to separate a polychromatic optical beam into its constituent wavelengths. Devices such as these generally are used in the telecommunications industry for the transmission of voice and similar signals over optical communications networks. The size of such devices generally is not an issue in the telecommunications industry and, thus, optical multiplexers and demultiplexers such as those disclosed in the above patents and others tend to be relatively large and bulky.
The past four decades have been a time during which microelectronics, including the integrated circuit chip, has advanced at exponential rates. Microelectronics has entered into almost all aspects of human life through the invention of small electronic devices such as watches, hearing aids, implantable cardiac pacemakers, pocket calculators, and personal computers. The advance of microelectronics has become the principal driving force of innovation in modern information technologies and high-density data communications such as fiber communications, global satellite communications, cellular phones, the Internet, and the World Wide Web. As microelectronics techniques advance, nano-electronics (feature scales on the order of 10−9 meters) are being realized.
Based on the current growth rate of data communication traffic, the microelectronic chip of 2010 likely will be an array of parallel processors consisting of at least 1024 channels with processing speeds of 40 Gigabytes per second (Gb/s) or faster for each channel. This pushes semiconductor technology towards gigascale and terascale integration with smaller component or feature sizes and larger chip sizes. At the same time, interconnections between circuit components on the chip must support the data transfer rates of 40 Gb/s or faster. As integrated circuit feature sizes continue to decrease and chip sizes continue to increase, interconnections formed of conventional electrical interconnects and switching technology are rapidly becoming a critical issue in the realization of microelectronic systems. It is believed that the maximum length of interconnection required for a chip is proportional to one half of the square root of the chip area. This parameter thus will be approximately constant while the circuit feature size and required interconnection data throughput scales down. As a consequence, the interconnection delay will be kept approximately constant while device delay is reduced as feature sizes are scaled down. The interconnection delay can even increase if chip size is scaled up. At some point in this scaling process, interconnection delay will dominate system speed; i.e., system speed will not be able to track increasing device speed performance due to the interconnection delay. Conventional conductor and semiconductor interconnects are not able to sustain the required future data rates of 40 Gb/s or higher. Thus conventional interconnects between features on future chips will be an insurmountable bottleneck to the throughput of high-density data communication systems and will be unworkable in future high-speed microelectronics.
To handle the unprecedented growth of data and telecommunications traffic, many novel transmission mechanisms have been proposed, including 3D structures with multiple levels of transistors and conventional interconnects, wireless RF interconnections using co-planar waveguides and capacitive couplers to obtain a “micro-area network on a chip,” and on chip optical interconnections. Of these proposals, optical interconnections, which has proven itself in large scale telecommunications networks, appears to hold the most promise. This is due to a number of factors including the fact that the propagation speed of an optical signal is independent of the number of electronic components that receive the signal, the fact that optical interconnections do not suffer mutual interference effects, and that optical interconnect paths can cross each other without significant interaction. As a result, optical interconnections between microchip features promises to enhance communication performance by providing larger fan-outs at higher bandwidths.
There are two major challenges to the introduction of optical interconnections to microelectronic data communication systems such as computer chips. First, the optical systems and the electronic systems have different architectures since they operate under different physical principals. Second, optical component technology on a micro- or nano-optical scale necessary for implementation of on-chip optical interconnects is not mature and it is costly. Thus, the key to successful application of optical interconnections to high-density microelectronic systems is to perform very effective integration of exceedingly small but highly efficient optical devices with increasingly smaller microelectronic circuitry components.
In order to maximize the potential of micro-optical interconnects for data communications, wavelength division multiplexing of multiple optical signals on a micro- or nano-scale will be employed just as it has been on a macro scale in the telecommunications industry. This requirement calls for exceedingly small optical multiplexers and demultiplexers for combining and separating discrete wavelength optical signals. Further, due to power and heat dissipation constraints present in a microelectronic circuit environment, these micro-optical multiplexers and demultiplexers must operate with virtually no optical transmission losses, otherwise the data throughput will be compromised. Finally, the micro-optical multiplexers and demultiplexers must be highly integrated with micro-optical transmitters for generating the optical signals to be multiplexed and transmitted and with micro-optical sensors or detectors for receiving demultiplexed optical signals. In addition, related interface circuitry will be required for transforming electronic signals from microcircuit components into optical signals and vice versa for integrating optical interconnection components with electronic CMOS microcircuit components, all on a micro- or nano-scale.
One type of optical diffraction grating capable of separating a multiplexed polychromatic optical signal into its constituent component beams with virtually no transmission loss is known as a blazed grating. Blazed gratings on a macroscopic scale are known and need not be described in detail here. U.S. Pat. No. 4,359,373 of Hammer, and U.S. Pat. No. 5,279,924 of Sakai et al. disclose and discuss blazed gratings in substantial detail and their disclosures are hereby incorporated fully by reference. Generally, however, a blazed grating is a type of diffraction grating characterized by an asymmetric groove structure wherein adjacent ridges are substantially triangular in cross section, forming an array of microprisms. Blazed gratings are extremely efficient and can be designed to divert or allocate nearly 100% of the power of an incident optical beam into a single diffracted order such as, for example, the +1 order. When an incident beam is a multiplexed polychromatic beam, each discrete wavelength component beam within the incident beam is diffracted at a different angle and thus the component beams are fanned out and separated, resulting in demultiplexing of the incident beam. Optical sensors can be positioned to intercept the discrete beams for detection and demodulation of data they carry. Since nearly 100% of the incident power is preserved by the blazed grating, the demultiplexing is accomplished with virtually no transmission loss, which translates to higher data throughput with an optical signal of a given power.
While blazed gratings have potential as highly efficient, compact, planar demultiplexers and waveguide couplers, they carry significant inherent problems in that the continuously varying profile of the microprism ridges are difficult and expensive to fabricate. Fabrication becomes an increasing problem as the size and scale of the grating is reduced until, at some threshold, known fabrication techniques such as ion beam etching simply are ineffective to produce the grating. At the micro- or nano-scales required for integrated micro-optical interconnections, no known fabrication technique is available.
Even if exceedingly small scale blazed gratings could be fabricated, there still is an inherent and inescapable practical lower limit to their size for demultiplexing applications. More specifically, as the period of the grating elements in a blazed grating becomes smaller and approaches the wavelength of the incident light, the blazed grating progressively becomes a so-called zero order grating. In other words, when the grating period is extremely small, and certainly when it is smaller than the wavelength of the incident light, i.e. when the period is sub-wavelength, a regular blazed grating allocates all of the transmitted light to the zero diffractive order rather than to the first or higher orders. Under such conditions, an incident light beam is not diffracted as it traverses the grating but, instead, passes straight through the grating regardless of its wavelength. However, optical demultiplexing fundamentally requires that light of different wavelengths be diffracted or fanned-out at different angles by a grating so that they are separated. Since a zero order grating passes each wavelength straight through, the different wavelengths are not separated and there is no separating or demultiplexing of a polychromatic optical signal. Thus, regular blazed gratings simply are not functional as optical demultiplexers on the micro- or nano-scale necessary for use in microelectronics data interconnections.
Accordingly, even though regular blazed gratings on a macro scale theoretically offer the performance characteristics necessary for use in integrated micro-optical interconnections, they are in fact not suitable for such applications for a variety of reasons as discussed above.
The performance characteristics of regular blazed gratings can be approached by so-called multi-level gratings wherein the continuously varying sloped surfaces of the grating elements of a regular blazed grating are simulated with multiple discrete surface levels or steps. According to theory, a multi-level grating with 16 levels or steps per grating element can deflect 99% of input beam power to a designated diffractive order. Such a multi-level grating is disclosed in U.S. Pat. No. 5,742,433 of Shiono et al. One problem with multi-level gratings is that multi-step fabrication techniques are required for their manufacture with the number of steps being proportional to the number of levels in the grating features. As a result, the critical alignment of the various levels of each grating element is exceedingly difficult to maintain, especially on the sub-wavelength scales required for microcircuit interconnections. Accordingly, multi-level gratings are not a practical solution to the problems with regular blazed gratings.
Binary blazed gratings have been developed as another alternative to regular blazed gratings. Essentially, a binary blazed grating is a grating in which the grating ridges are all at a single level and the grating troughs are at a single level (i.e. two steps), but the ridge width, trough width, and/or spacing between grating elements varies to create localized subwavelength, submicrometer grating features within the grating period. Fundamental research on the design and optimization of binary blazed gratings has been conducted by the inventor of the present invention and others. This research is presented in Z. Zhou and T. J. Drabik, Optimized Binary, Phase-only, Diffractive Element with Subwavelength Features for 1.55 μm, J. Opt. Soc. Am. A/Vol. 12, No. 5/May 1995; and Z. Zhou and N. Hartman, Binary Blazed Grating for High Efficient Waveguide Coupling, SPIE Vol. 2891, 1996. The theory and optimization of a binary blazed grating as an alternative to a regular or linear blazed grating is presented in substantial detail in these papers and thus need not be repeated here. Instead, the disclosures of these papers are hereby incorporated by reference as if fully set forth herein.
Binary blazed gratings have been shown to exhibit transmission efficiencies when diffracting light into the first or higher diffractive orders that approaches that of a regular blazed grating. However, binary blazed gratings have several inherent advantages both over regular or linear blazed gratings and over multi-level gratings. Specifically, the subwavelength grating features of a binary blazed grating can be fabricated relatively easily and in a single step with existing fabrication techniques. Further, and most significantly for the present invention, binary blazed gratings do not become zero order gratings at subwavelength scales as do regular blazed gratings. In other words, a binary blazed grating continues to allocate a very high percentage of the power of an incident light beam into the first or a higher diffractive order, even when the grating elements are smaller than the wavelength of the incident beam.
In view of the foregoing, it will be seen that a need exists for an integrated optical multiplexer and demultiplexer for use in wavelength division transmission of information that is downwardly scalable to be incorporated into micro-electronic devices as optical interconnections between electronic components. The integration should include signal conditioning circuitry for converting transmitted information between the optical and electronic domains for integration with CMOS circuit components. Further, the multiplexing and demultiplexing functions should be performed with near perfect transmission efficiency similar to that obtainable on a macro scale with a regular blazed grating to preserve optical power, minimize heat generation, and maximize information throughput. It is to the provision of such a device that the present invention is primarily directed.